1. Field of the Invention
The present invention relates to a polycrystalline Si (silicon)-used thin film transistor substrate applicable to a driver-integrated type display unit and a method for manufacturing thereof.
The present invention is effectively applicable to thin film transistor substrates, and particularly, low temperature polycrystalline Si technology-used thin film transistor substrates, used in driver-integrated type liquid crystal display units and organic electroluminescence display units.
2. Related Arts
Aimed at higher definition and lower cost liquid crystal display units, a thin film transistor (hereinafter abbreviated as TFT)-used driver is formed on a substrate. These TFTs are fabricated from a polycrystalline Si film formed at low temperatures (hereinafter denoted simply as a low temperature polycrystalline Si film). In the case of mobile information terminals, it is especially needed to reduce the occupation area and power consumption of their driver.
Directed to these and other needs, many techniques have been proposed. They are largely classified into two categories. One is to fabricate on the same substrate both high voltage thin film transistors needed to drive liquid crystal and high performance thin film transistors for the driver while the other is to use a low temperature polycrystalline Si film to fabricate thin film transistors constituting the electronic circuit of the display unit. A method for fabricating on the same substrate both high voltage thin film transistors needed to drive liquid crystal and high performance thin film transistors for the driver is disclosed in, for example, Japanese Patent Laid-open No. 5-335573. In the case of N-type thin film transistors, a lightly doped drain (LDD) region is formed at the end of the gate in order to raise the withstand voltage and reduce the off current. For example, in Japanese Patent Laid-open No. 11-163366, a method is disclosed which forms a LDD region at the end of the gate in a self-aligned manner without misalignment. Meanwhile, a display unit in which organic electroluminescence elements are driven by thin film transistors made from a low temperature polycrystalline Si film was disclosed at ASIA DISPLAY/IDW'01, Proceedings, p. 319.
Prior art TFTs, with which the present invention is concerned, involves the following drawback. Since a plurality of transistor regions is formed, a dopant must be implanted into the semiconductor film through gate insulation films which have different thicknesses. In this process, the dopant implanted depth varies depending on the thickness of the gate insulation film as a matter of course. Accordingly, to form impurity regions which require different implantation depths, implantation must be done in several stages while changing the acceleration voltage, resulting in an increased number of production steps. Meanwhile, implanting a dopant into the semiconductor film before forming a gate insulation film is likely to cause misalignment between the gate and doped regions although the number of production steps can be suppressed. Therefore, this method adds an increased parasitic capacitance to the transistor.
In addition, forming both high voltage thin film transistors with LDDs and high performance thin film transistors with no LDDs on the same substrate requires another mask to selectively form LDD regions, resulting in increased production steps.
With this situation behind, it is a first object of the present invention to provide a simple manufacturing method for forming on the same substrate both high voltage thin film transistors with LDDs, suitable for driving liquid crystal or the like, and high performance thin film transistors capable of low voltage driving.
If a polycrystalline Si film is used to form thin film transistors in a driver-integrated type display unit, each pixel must have a large capacitance to retain the image signal since the off current of TFT is large. Disadvantageously, this makes it impossible to raise the aperture ratio. It is another object of the present invention to provide a high aperture ratio and low power consumption display unit of this type by reducing the area of capacitance. To achieve this object, the present invention provides a thin film transistor substrate having a higher aperture ratio than conventional ones.